1. Field of the Invention
The present invention relates to porphyrin metal complexes which can serve as two-photon absorption materials, and derivatives thereof. More specifically, the present invention relates to porphyrin arrays which exhibit large two-photon absorption, and having, as a structural unit, bis(imidazolylporphyrin metal complex) linked with an acetylic bond, and derivatives thereof. The present invention also relates to porphyrin arrays having, as a structural unit, the bis(imidazolylporphyrin metal complex) that is further fixed with covalent bonds.
2. Description of the Related Art
It is expected that a two-photon absorption material will be applied to various applications such as three-dimensional optical memory, photodynamic therapy, optical limiting materials, and a two-photon microscopy. Among these applications, three-dimensional optical memory which enables ultrafast reading and writing with a very high density, and photodynamic therapy in which only cancer cells are selectively attacked by laser for medical treatment, are particularly important in view of a great contribution to the society.
In designing an organic compound exhibiting a relatively large two-photon absorption cross section, it is presumably very important that π-electron conjugated system is expanded thereby the overlap of the orbitals is increased, and polarization of the molecule is enhanced by appropriately combining an electron donor and an electron acceptor. As porphyrin is a ring-shaped tetra pyrrole in which four pyrrole nucleuses are cross-linked by four methine groups, and has a large conjugated system including 18 π-electrons, porphyrin is one of the promising candidates of the two-photon absorption materials.
The phenomenon of two-photon absorption itself has been known for years, but advanced research on two-photon absorption has finally begun since Jean-Luc Brédas et al. revealed the relationship between the molecular structure and the mechanism of two-photon absorption in 1998 (Science, 281, 1653 (1998)). However, there are few reports on the two-photon absorption of porphyrin. Quite recently, Anderson reported that a one-dimensional linear porphyrin polymer, in which porphyrins are linked with butadiyne bonds, exhibits a relatively large two-photon absorption cross section (J. Am. Chem. Soc., 124, 9712 (2002)). However, the length of the polymer cannot be adjusted, because covalent bonds are used for the linkages in the porphyrin polymer. Further, as introduction of hetero metals thereto and introduction of electron donors/electron acceptors to the terminal groups thereof is difficult, the two-photon absorption property of the porphyrin polymer cannot be further improved. Yet further, in Anderson's case, the two-photon absorption property is measured on the time scale of picoseconds, and the analysis of two-photon absorption in the order of femtoseconds, which essentially requires for an ultra fast recording, has not been carried out. As long as covalent bonds are used for the linkage of porphyrins, introduction of electron donors or electron acceptors, which enables large two-photon absorption, to a one-dimensional linear porphyrin array, was difficult. Therefore, in the conventional method of preparing porphyrin arrays, there is no prospect of achieving large two-photon absorption.